1. Technical Field
This invention relates to thermally responsive indicating devices. More particularly, it relates to devices that provide a signal upon the attainment of specified temperatures.
2. Discussion of the Art
Thermally responsive indicators are useful in a variety of fields for providing a visual indication of the attainment of a specified temperature. For example, U.S. Pat. No. 4,083,364 discloses a thermal indicator mounted through the skull of an animal to detect the presence of an elevated body temperature. German Patent No. 3,229,020 discloses a thermally responsive indicator that is designed to be mounted on an electrical conductor. The device provides a visual indication of the attainment of an elevated temperature in the conductor. U.S. Pat. No. 4,818,119 discloses a railroad wheel bearing bolt with an axially extending bore into which a heat sensor and indicator are inserted. The indicator is exposed when a specified temperature is attained. U.S. Pat. No. 4,289,088 discloses a sterility indicating device for use in a steam autoclave.
Thermally responsive indicating devices are especially useful in preparing food products, particularly meat and fowl. Such devices can be used to indicate the elevated temperature of the interior of the food product, rather than the temperature of the exterior thereof. By indicating the attainment of a specified internal temperature of the food product, the device can signal when the food product is organoleptically acceptable. These devices must be sufficiently accurate to prevent undercooking or overcooking, which not only may significantly diminish the palatability of the food, but may also even render the food dangerous to eat, as in the case of undercooked pork.
Several such devices for use in cooking food are known. Examples of early indicating devices are described in U.S. Pat. Nos. 945,978 and 1,509,110, each of which allows a plunger biased by a spring to be released into an extended position upon attaining a specified temperature. Retaining means, which is typically a fusible material, holds the plunger in a retracted position until the fusible material yields, at which time a spring urges the plunger into an extended position. When in the extended position, the plunger provides a visual indication to the user that the food is "done." To further enhance the visibility of the plunger when it is in the extended position, a cap may be attached to the end thereof.
The material of the retaining means has typically comprised metal alloys, as in U.S. Pat. Nos. 3,140,611, 3,682,130, 3,693,579 and 3,713,416. Indicators that use such alloys include the "Vue-Temp" brand and "Pop-Rite" brand pop-up timers, manufactured by the Volk Development Company of Turlock, Calif. 95381. While devices employing such retaining materials have proven useful, they have certain drawbacks. For example, such alloys are typically prepared from toxic metallic substances such as bismuth, lead, cadmium, tin, and other materials. Furthermore, metal alloys have high density. Thus, a given volume of retaining material requires a significant amount of alloy, thereby adding to the expense of the device.
Organic compounds, such as waxes, have been suggested as replacements for metal alloys. However, waxes typically yield over a relatively wide temperature range, and, if Composed of a mixture of different materials, can have multiple melting temperatures, thereby resulting in a premature or a delayed signal. This, as mentioned above, is undesirable because the food item could either be undercooked or overcooked, respectively.
A device that attempts to overcome the problems exhibited by alloy and wax retaining materials is described in U.S. Pat. No. 4,170,956. This patent discloses a nitrogen-containing organic retaining material. Devices using these nitrogen-containing materials have been sold in the United States under the name "Dun-Rite" brand pop-up timers by Minnesota Mining and Manufacturing Company of St. Paul, Minn. The nitrogen-based materials used in these devices do not exhibit the toxicity problems of their alloy predecessors and exhibit better release characteristics than waxes. However, the nitrogen-based materials disclosed in this patent also have certain drawbacks.
For example, the nitrogen-based materials of this patent are very pure when they are prepared. Melting temperature (and therefore yield temperature) tends to increase with increased purity, and the materials of this patent melt at approximately 84.9.degree. C. when they are tested in their purest state. A turkey is considered "done" when the interior temperature thereof reaches approximately 82.2.degree. C. Consequently, the nitrogen-based retaining material must be mixed with a foreign substance to reduce its purity. The use of foreign substances adds unnecessary expense. Salts, which are added to the nitrogen-based material to clean it, tend to solidify to form gel globules in the material, thereby requiring that the material be strained. The straining process often must be conducted repeatedly, thereby increasing the time and cost of production. The yield is reduced, and, consequently, a greater amount of material must be manufactured and processed to obtain a given amount of retaining material.